I 1 8 READINGS IN BIOLOGICAL SCIENCE 



cyclic action of heart muscle in the following respect. If the nerves of 

 the muscles of breathing are severed, the movements cease at once. Their 

 contractions are entirely dependent upon rhythmic activation through 

 their external or extrinsic nerves. The beating of the heart, on the other 

 hand, continues even after all its nerves are cut. The rhythmicity and 

 automaticity are inherent in the heart itself. In fact, if the organ is com- 

 pletely removed from the body, it will continue to beat for some time. 

 Nor is the integrity of the organ itself required for this automaticity. A 

 bit of the heart muscle, cut off from the organ, may continue to contract 

 rhythmically. Even in tissue culture, microscopic pieces of cardiac tissue 

 sometimes continue to pulsate. 



During each cardiac cycle, characteristic sounds are produced by the 

 heart. They can be heard by placing the ear against the chest over the 

 heart, or by leading the sounds to the ears through the tubes of an instru- 

 ment called a stethoscope, the receiving end of which is placed over the 

 heart. In each cycle two distinct sounds are heard, termed the "first" and 

 "second" heart sounds. The first is low pitched, the second is sharper, 

 louder, higher pitched, and of short duration. The only way to appreciate 

 what these sounds are Uke is to listen to them. They roughly resemble the 

 sounds of the syllables "lubb-dup." 



The second sound is known to be due to vibrations set up by the sudden 

 closure of the semilunar valves very soon after the beginning of ventricular 

 diastole. Experimental injury to these valves modifies the sound, cor- 

 responding to faulty function. If they are slit open, for example, so that 

 they do not close tightly in diastole, and blood therefore leaks back into 

 the ventricles, the second sound is of a rather soft hissing character, called 

 a "murmur." Instead of the normal "lubb-dup," there is heard "lubb- 

 shhh." 



This finding is of significance not only in indicating the cause of the 

 second sound but also in detecting the existence of defective valves. If 

 the valves are damaged by syphilis, for instance, the presence of the injury 

 may be detected by the abnormality of the second sound. 



The first heart sound is of more complex origin. It is partly due to 

 vibrations set up by closure of the auriculo-ventricular valves at the be- 

 ginning of ventricular systole; for, if these valves are damaged experi- 

 mentally, or by disease, the sound is somewhat modified. However, the 

 sound persists even though, for the moment, the flow of blood through 

 the heart is stopped experimentally. During such time, of course, all valve 

 action ceases. The second sound is eliminated, but the first persists in 

 modified form. It is thought that for the most part this sound is caused by 

 vibrations set up by the contractions of the muscle fibers. Even skeletal 

 muscle fibers produce such vibrations, giving rise to sounds which can be 

 heard by placing a stethoscope on any contracting muscle. 



Throughout the vascular system all the vessels are hollow tubes of dif- 



